Not applicable.
Not applicable.
The present invention relates to an evaporative burner, for example, such as used for heating devices in motor vehicles.
Patent document WO 98/49494 discloses an evaporative burner, in which a porous evaporative medium, for example nonwoven material, is arranged in the floor region of a combustion chamber. Liquid fuel is conducted into this porous evaporative medium to be distributed in the evaporative medium by capillary action. The fuel evaporates on the side toward the combustion chamber, so that an ignitable or combustible mixture is formed on the side toward the combustion chamber by the accumulation of fuel vapor and combustion air in the region of the combustion chamber. A heating device is furthermore provided that includes a glow ignition pin projecting into the region of the combustion chamber. By heating the glow ignition pin, a high temperature is produced in its surroundings, such that the ignitable mixture in this region ignites and thereupon propagates into the region of the combustion chamber.
An evaporative burner is also known from German patent document DE 32 33 319 A1 in which a porous material is again provided in the floor region of a combustion chamber for the distribution and evaporation of fuel. A heating device constituted in the manner of a heating coil is provided on the side of the porous medium lying open toward the combustion chamber, and when current is applied can produce in the region of the porous medium the temperatures of about 1,100xc2x0 C. required for combustion.
Such evaporative burners known from the prior art have the disadvantage that they require a comparatively long time to reach a high heating power, and the time is distinctly longer than that required, for example, by pressure pulverizers, air atomizer burners, or ultrasonic atomizer burners. A substantial reason for this is that energy for the evaporation of further fuel is also withdrawn from the flame arising from ignition, and prevents rapid flame propagation into the combustion chamber, particularly at low external temperatures and with large component masses with comparatively good thermal conduction. This disadvantage of evaporative burners that are basically of interest due to their cost-effective construction is of little effect when they are used as auxiliary (stationary) heaters, for example. Here, the spontaneous production of comparatively high temperatures is not a matter of prime importance. However, it is another matter when such a burner is used as a supplementary heater, which is effective particularly for the cold start of an engine at low environmental temperatures. In this case, it is required that a very high heating power of the supplementary heater can be provided in a very short time, in order above all to reduce the pollutant emission in the starting phase of a drive assembly heated in this manner.
The present invention has as its object to provide an evaporative burner in which the operating phase of high heating power can be attained more rapidly.
According to the present invention, in order to attain this object an evaporative burner is provided, having an evaporative medium for feeding fuel vapor into a combustion chamber, a first heating device having at least one ignition heating element projecting at least with its heating region into the combustion chamber, for igniting the fuel vapor present in the combustion chamber, and also a second heating device, comprising at least one evaporative heating element associated with the evaporative medium in order to affect on its evaporation characteristics.
The present invention eliminates the prior art disadvantage by providing respective separate heating devices, one for ignition and the other for evaporating the fuel supplied in liquid form. These can be respectively optimally matched to what is required as regards the temperatures that they produce and the heating power required therefor. The rate of evaporation is increased by preheating the fuel to be evaporated, the withdrawal of heat energy from the propagating flame nevertheless being prevented. Flame propagation in the starting phase of such an evaporative burner clearly takes place more quickly, so that full load operation is finally also clearly attained more rapidly than with the evaporative burners known from the prior art.
In order to not expose the evaporative heating element, used solely to preheat the fuel to be evaporated, to the comparatively high temperatures prevailing in the combustion chamber, the at least one evaporative heating element is arranged on a side of the evaporative medium remote from the combustion chamber. This can be achieved, for example, by providing the evaporative medium on an evaporative medium support, and by arranging at least one evaporative heating element between the evaporative medium and the evaporative medium support. A still further protection of the evaporative heating element from excessively high temperatures can be achieved in that the evaporative medium is provided on a evaporative medium support and that the at least one evaporative heating element is provided on a side of the evaporative medium support remote from the evaporative medium.
In the evaporative burner according to the invention, there is furthermore provided a fuel feed channel arrangement for introducing the liquid fuel into the evaporative medium. In order to achieve an approximately uniform combustion characteristic over the whole combustion chamber, the fuel feed channel arrangement is constructed so as to distribute the liquid fuel over the evaporative medium. This can be attained, for example, in that the fuel feed channel arrangement has at least one annular channel region and/or at least one radial channel region going out from a fuel feed duct substantially radially in the evaporative medium and/or in an evaporative medium support.
The evaporative burner according to the invention has, for providing the ignitable mixture in the combustion chamber, an air supply channel arrangement for supplying air to the combustion chamber for combustion with the fuel vapor. For this purpose it can for example be provided that the air supply channel arrangement has at least one air inlet opening in the wall bounding the combustion chamber and open toward the combustion chamber.
In order to also deliver the combustion air required for ignition, simultaneously with the fuel vapor coming from the evaporative medium, into that spatial region in which the ignition occurs, the air supply channel arrangement has at least one air inlet opening which is open to the evaporative medium. For this purpose it can further be provided that the air inlet opening has at least one air supply channel region passing through the evaporative medium.
Since the heat removal occurring in the region of an evaporative burner is an important parameter affecting rapid flame propagation, according to a further aspect of the invention a better thermal insulation, and thus a further acceleration of flame propagation, can be provided for in that the at least one evaporative heating element and the evaporative medium are provided on an evaporative medium support made of ceramic material.
The evaporative medium can comprise porous material that can be of multilayer construction in order to achieve as rapid as possible a dispersion of the liquid fuel in the evaporative medium itself and then for the evaporation of the distributed liquid fuel. A nonwoven material can be used here, for example.
A general problem that arises in the operation of evaporative burners is in the first place the required high variability of the burner power. For example, a ratio of maximum to minimum burner power of at least 4:1 is required. In the second place, evaporative burners of this kind are to be operated with many different fuels, or with fuels of different quality. For example, besides being able to use conventional diesel fuel, it is of course also required here to be able to use winter diesel or arctic diesel. Also of increasing importance are natural-based fuels such as biodiesel produced from rape oil, and also fatty acid methyl ester fuels obtained by the transesterification of oils. The consequence of the use of often even unspecified fuels, particularly in connection with the high variability of the burner power, is the danger of deposits arising during combustion in that region in which the combustion takes place, thus particularly in the region of the combustion chamber, or in that region in which the evaporation of the basically liquid fuel takes place. One reason for this, among others, is that the evaporation does not always take place under optimum conditions, such as, for example, optimum evaporation temperature and optimum oxygen supply. The formation of deposits, which in general can be regenerated, i.e., are combustible deposits, impairs the operating characteristic of such an evaporative burner, whereby the maximum operating lifetime can also be limited.
According to a further aspect of the present invention, an evaporative burner has a cleaning arrangement for the removal of deposits which are deposited in the region of the combustion chamber during operation.
The provision of the cleaning arrangement can ensure that deposits or contamination produced or precipitated in the region of the combustion chamber are removed again, so that the evaporative burner can again be operated with improved efficiency.
Since the deposits forming in combustion operation are, as above-mentioned, in general themselves combustible, according to a further aspect of the present invention the cleaning arrangement includes a heating arrangement by means of which a temperature in the region of, or above, a burning-off temperature of the deposits can be produced.
Since, as already previously stated, that region in which the evaporation takes place is above all critical as regards the precipitation of deposits, it is provided, according to a further aspect of the present invention, that the heating arrangement is constituted for the production of a temperature in the region of, or above, a burning-off temperature of the deposits, at least in the region of the evaporative medium.
Particularly when the evaporative medium is provided with its own heating device, according to a further aspect of the present invention, this heating device also forms the heating arrangement used for cleaning. According to whether a normal evaporative operation or a burning-off operation for cleaning is provided, this heating device can then be operated with different heating power, in order to produce correspondingly different temperatures, which are suitable for the different operating phases.
According to a further aspect, the present invention relates to a cleaning process for the cleaning of a heating burner, in particular of an evaporative burner as was previously described, in which process, by the activation of a heating arrangement, deposits on a wall surrounding a combustion chamber are heated to a temperature in the region of, or above, the burning-off temperature of the deposits, and are burned off.
It is then provided that the cleaning process is carried out when the heating burner is not in a state of heating operation. Since various system components cooperate in normal heating operation so that fuel and oxygen are introduced in a ratio suitable for combustion, this measure according to the invention can ensure that oxygen which would per se be required for the normal combustion of the injected or evaporated fuel is not used for the burning-off of the deposits by a burning-off taking place during a heating operation phase, and thus becomes no longer available for combustion. An impairment of the normal operation can thus be avoided.
According to the present invention, the cleaning process is carried out following on a heating operation phase of the heating burner. The advantage of this measure is that the various system components are already heated, following on a normal heating operation state, so that the heating power necessary for burning off the contamination or deposits can be correspondingly reduced.
In order to ensure, even over a longer operating lifetime, that the operating characteristic of a heating burner is impaired as little possible by the formation of deposits, the process is carried out after a predetermined operating period of the heating burner. The time is monitored for which the heating device has been operated, possibly since the last cleaning. If a given maximum number of operating hours is reached here, the cleaning process according to the invention is carried out again.
In carrying out this cleaning process, the heating arrangement can then be driven with a mark/space ratio of less than unity. The advantage of this measure is that the heating power can be regulated in a simple manner by the cyclic driving of the heating device, without having to be dependent on the available supply voltage or being substantially limited by this.
In the operation of evaporative burners, it is important to know whether a metering pump device that introduces fuel into the combustion chamber is operating correctly, or whether fuel is present in the evaporative burner, in order to start or carry out the combustion in the correct manner. A method for this purpose is known, for example, from German patent document DE 198 59 319 A1, in which the excitation current of the metering pump is monitored, and based on the evaluation of this electrical current flowing through the metering pump, it is concluded whether or not the latter is operating correctly. However, it is difficult, for example, to also recognize defects which possibly do not reside in the metering pump itself, but only arise in the connecting region between the metering pump and the combustion chamber. Furthermore, this monitoring process is very expensive, because of the manufacturing tolerances in the manufacture of the metering pumps, and can be used only with comparatively low precision.
In order to decide with increased precision whether an evaporative burner is being correctly supplied with fuel, according to a further aspect of the present invention, the evaporative burner can have a control device by means of which the heating power at least of the second heating device can be adjusted, with the monitoring module monitoring the heating power and/or the required heating power of the second heating device and, based on the result of monitoring, detecting the presence of fuel evaporation.
The present invention makes use in this connection of the fact that the power of the heating device supporting the evaporation has to be increased in order to maintain the same temperature, when there is a transition from a state in which no evaporation is present to a state in which evaporation is present, because of the energy required for the evaporation of fuel and withdrawn from the surroundings. There would otherwise occur a cooling of that region in which the evaporation takes place. The present invention makes use of this change in the driving characteristic, or of the required driving characteristic, for this heating device in order to sense when the transition into the evaporation state occurs.
Furthermore, the evaporating heating element comprises an electrically operated heating element with an electrical resistance, which increases with temperature.
The present invention furthermore relates to a process for monitoring the fuel supply to an evaporative burner; this process can in particular be used in an evaporative burner according to the invention. This evaporative burner comprises a heating device provided for supporting the evaporation of fuel. In the process, it is determined, depending on the heating power of the heating device and/or on a change in the heating power of the heating device and/or on a required change in the heating power of the heating device, whether evaporation of fuel is present in a combustion chamber of the evaporative burner.
The procedure can, for example, be that the presence of the evaporation of fuel can be detected when there is a rise in heating power, and/or higher required heating power, during the operation of the heating device.
Since it is of considerable importance for the initial operation of an evaporative burner to detect when evaporated fuel is available, in order to then release further procedures, according to a further aspect of the invention for an ignition process of the evaporative burner, the heating device is operated in a first operating phase with higher heating power, in the region of a maximum heating power; in a subsequent, second operating phase, the heating device is operated with reduced, preferably heating power, and in a further subsequent, third operating phase, the heating device is operated with a heating power which is raised again and is increasing, the presence of fuel evaporation being detected at or after the transition into the third operating phase. When evaporation of fuel is detected, a heating device that supports the ignition of the evaporated fuel is activated.
If an evaporative burner is set out of action, which can occur by the deactivation of a heating device supporting the combustion and adjustment of the fuel supply, it is advantageous to ensure that fuel residues still present in the evaporative burner are completely ejected. This can for example take place in that a heating device supporting the evaporation is activated and the still present fuel is volatilized. Because of the above-described physical effect that energy is required for producing fuel evaporation, and is made available by the corresponding excitation of the associated heating device, according to the invention when the heating power, or required heating power, of the heating device supporting evaporation decreases, it is detected that further fuel is no longer available for evaporation. The reason for this is also again that when no further fuel is available, heat of evaporation no longer has to be made available, so that in order to maintain a predetermined temperature the heating power provided by the corresponding heating device can be reduced. This reduction of the heating power or of the required heating power can be made use of as a decision criterion.